1. Field of the Invention
The present invention relates to a stabilizer or dispersing agent for use in a suspension polymerization, and more particularly to a stabilizer or dispersing agent for use in a suspension polymerization of a vinyl compound and a process for preparing a polymer of vinyl compound by the use of said stabilizer or dispersing agent.
2. Prior Art
In the process for suspending vinyl chloride monomers in water and then polymerizing the same in the presence of a catalyst which is soluble in the monomers, a suspending agent or stabilizer has hitherto been added to the reaction mixture in order to disperse the oily droplets of monomer stably in water and to prevent the formed polyvinyl chloride particles from adhering with each other. For this purpose, polyvinyl alcohol, cellulose derivatives and surfactants are commonly used.
It is well known that polyvinyl alcohol is most frequently used for this purpose, since it functions excellently when used as the suspending agent or stabilizer of this kind. However, the polyvinyl chloride prepared by the use of an unmodified polyvinyl alcohol is poor in porosity which results in reduction of the absorption speed and the amount of a plasticizer, leading to unsatisfactory workability in the later processing steps (for example, gelation at a later processing step is retarded). A further disadvantage of the polyvinyl chloride prepared by the use of an unmodified polyvinyl alcohol resides in that it is difficult to evaporate residual monomers contained in the formed polyvinyl chloride particles due to their poor porosities, resulting in an increase of monomers remaining in the polyvinyl chloride particles.
In order to overcome the aforementioned disadvantages, it has been proposed to use a different kind or combination of dispersing agents to change the interfacial tension and/or the viscosity of the polymerization system to improve the dispersibility thereby to change the shape or size of the formed particles to obtain porous particles. Examples of the previously proposed dispersing agents are polyvinyl alcohol having a hydrolysis degree of 60 to 90 mol%, a modified polyvinyl alcohol including carbonyl groups and having a hydrolysis degree of 60 to 90 mol% (Japanese Patent Provisional Publication No. 31090/1979), and a modified polyvinyl alcohol obtained by partially hydrolyzing a copolymer prepared from vinyl acetate and an unsaturated monocarboxylic acid (Japanese Patent Publication No. 13966/1972). However, these known dispersing agents or stabilizers are not satisfactory in that coarse polyvinyl chloride particles, which are poor in porosity, are formed due to poor dispersibility of the agent, minute polyvinyl chloride particles having too low apparent density are formed due to excessive dispersibility of the agent, or minute polyvinyl chloride particles, which are poor in porosity, are formed.
A further proposal has been made by Japanese Patent Publication No. 7540/1956, in which a partially hydrolyzed product of a copolymer of vinyl acetate and maleic acid anhydride is used as a stabilizer for the suspension. This stabilizer includes more than 40 mol% of maleic acid anhydride and a hydrolysis degree of 1 to 15 mol%. However, since the dispersive power of this stabilizer is not improved that much, the formed polyvinyl chloride particles are still coarse and poor in porosity and thus inferior in absorption of a plasticizer.
Monomers contained in polyvinyl chloride particles are conventionally removed by heating a slurry after the completion of polymerization to recover the monomers under a reduced pressure or by controlling the conditions at the drying step. These conventional methods have been proposed based on the fact that monomers can be easily removed from the polymer particles by heating. However, since polyvinyl chloride tends to be decomposed by the thermal hysteresis, the heating temperature and time are limited. As a consequence, inconsistency occurs in removal of monomers. If monomers are removed at a low temperature, the production efficiency is lowered. On the contrary, if monomers are removed at a high temperature in order to improve the production efficiency, the polyvinyl chloride is decomposed and colored.
There have been methods proposed for producing polyvinyl chloride which resist decomposition and coloring under thermal hysteresis, wherein metallic salts of lower aliphatic carboxylic acids, metallic salts of acrylic acid or bisphenol A are added in the slurry during or after the polymerization step. (Reference should be made to Japanese Patent Provisional Publication No. 66584/1975 and Japanese Patent Publication No. 25277/1976.) Furthermore, many proposals have been made to produce a vinyl chloride polymer system from which monomers are easily removed. (See Japanese Patent Provisional Publication Nos. 21090/1977, 152988/1977 and 102988/1978.). However, the methods disclosed in these earlier publications are not satisfactory.
It is also known to use polyvinyl alcohol and an alkyl cellulose in combination as a dispersing agent or stabilizer. However, satisfactory dispersing effect cannot be attained even when they are added prior to polymerization, since the mechanisms of action thereof at the polymerization step are different from each other. (See Japanese Patent Provisional Publication Nos. 47991/1976 and 5886/1977.) Although the methods of adding polyvinyl alcohol and an alkyl cellulose separately at respective stages of polymerization are known, these methods are disadvantageous in that a special device or a complicated operation is necessitated. (See Japanese Patent Publication Nos. 21670/1976 and 21671/1976.)